A polyvinyl alcohol film is excellent in gas barrier property, but insufficient in moisture resistance and water resistance and has great dependence of gas barrier property on humidity. Therefore, this film tends to markedly lower its gas barrier property depending on humidity. An ethylene-vinyl alcohol copolymer film is relatively good in gas barrier property and water resistance, but has great dependence of gas barrier property on humidity. A poly(meth)acrylic acid film exhibits excellent gas barrier property under dry conditions such as a relative humidity of 0%, but is poor in moisture resistance, water resistance, hot water resistance and water vapor resistance. In particular, its gas barrier property is markedly deteriorated under high-humidity conditions.
There have heretofore been proposed processes for providing a film excellent in gas barrier property, water resistance and hot water resistance and little in dependence of gas barrier property on humidity by subjecting a coating film formed from a mixture of poly(meth)acrylic acid or a partially neutralized product thereof and polyvinyl alcohol or a saccharide to a heat treatment (for example, Japanese Patent Nos. 2736600, 2811540, 3203287 and 3340780).
According to these processes, however, it is necessary to subject the coating film formed of the mixture to a heat treatment over a relatively long period of time at a high temperature of at least 100° C. for obtaining a gas barrier film. In addition, in the film described above, an ester bond is formed between both polymers by the heat treatment. As a result, the film undergoes high crosslinking, so that it is difficult to scrap or regenerate the film after use.
There has been proposed a process for producing a gas barrier film improved in hot water resistance and water vapor resistance by subjecting a coating film formed of a mixture of poly(meth)acrylic acid and polyvinyl alcohol or a saccharide to a heat treatment to form a film and then subjecting the film to a dipping treatment in a medium containing an alkali metal or alkaline earth metal to introduce an ionic bond between poly(meth)acrylic acid and the metal (for example, Japanese Patent Application Laid-Open No. 10-237180).
There has been proposed a process for producing a film excellent in gas barrier property, hot water resistance and water vapor resistance by forming a metal compound-containing layer on the surface of a coating film formed from a mixture of poly(meth)acrylic acid or a partially neutralized product thereof and polyvinyl alcohol or a saccharide to form an ionic bond by migration of the metal compound into the coating film (for example, Japanese Patent Application Laid-Open No. 2000-931).
Into the respective films disclosed in Japanese Patent Application Laid-Open Nos. 10-237180 and 2000-931, an ionically crosslinked (also referred to as metallically crosslinked) structure by an ionic bond is introduced in addition to a crosslinked structure by an ester bond formed by the heat treatment.
However, the processes for introducing the ionic bond making use of the metal compound, which is disclosed in Japanese Patent Application Laid-Open Nos. 10-237180 and 2000-931, also require to heat-treating the coating film having the above-described composition under high-temperature conditions. In addition, such a film is difficult to scrap or regenerate because the film has the crosslinked structure by the ester bond.
On the other hand, there has been proposed a moistureproof film having a laminate structure that a polyvinyl alcohol film, on at least one surface of which a thin silicon oxide film is formed, and a transparent film containing at least one transparent plastic film having a thin silicon oxide film on at least one surface thereof are joined to each other (for example, Japanese Patent Application Laid-Open No. 2-258251). The thin silicon oxide film is formed by vapor-depositing silicon oxide on the respective films. Such a deposition film of silicon oxide tends to produce minute cracks by flexing. Oxygen gas and moisture permeate through the minute cracks. Therefore, the gas barrier property and moisture proofness of this moistureproof film are deteriorated when the film is subjected to flex fatigue.
Not only the deposition film of silicon oxide, but also a plastic film with a metal foil such as an aluminum foil laminated thereon produces cracks by folding processing and flex fatigue. Therefore, such a film does not avoid the deterioration of gas barrier property. Thus, a multi-layer film having the deposition film of silicon oxide or aluminum foil is limited in forming and processing or use.